Shadow is the absence of light, likewise there is also the good, only in relation to the evil. If nothing were beautiful, then nothing would be ugly. We must learn to understand that good and evil belong together and only create each other.
It is important to understand that there will always be ups and downs in life, that there will be joy and sorrow. It is equally important to understand that these are not two different things, but two aspects of the same thing. Sickness and health are not two completely different things, they are both aspects of the physical condition. If we were never sick, then we wouldn't even know what health was. It is also the case that if we were always sick, we would not know what health is at all.
It is exactly the same with the good and the evil. Of course, it would be desirable if the evil in the world was kept to a minimum. However, only the existence of the evil, respectively the bad, makes possible that the really good can move into this world. Here I am not necessarily talking about man-made evil. Any kind of tragedy can be evil. Whether it is a traffic accident, a sudden illness, the hunger that many people suffer, or loneliness. All of these can be forms of evil and bad. However, without them, it would not be possible to truly enjoy one's health, food or company.
We should also remember that life never goes in one direction forever. In times when we are not doing well and may not be in particularly good health or feel alone, we should redirect our focus. In these times it usually becomes crystal clear what we want or need. Then, when we are again in times when we are doing better, we should practice gratitude. We should keep in mind that it will not always be like this and that it is even more advisable to enjoy the good times. If you are more interested in the ups and downs in life then take a look at this article over Alan Watts' statement on the subject.
To briefly summarize:
If you are not doing well, remember that everything in life is in permanent change.
If you are doing well, remember that everything in life is in permanent change.
Evil is the necessary evil to bring forth the greatest possible good in us. The active expulsion of all evil or bad brings forth the truly good in us. For it is only when we sacrifice ourselves that we can do real good. An example of such a great feat of goodness would be Oskar Schindler, who risked his own life during the atrocities of the Nazi regime to save some 1200 lives of Jewish forced laborers. Or to put it less dramatically: You can only help someone up if they have fallen before it. This selfless helping up without gaining a personal advantage is the good in the world that can never exist without evil.
Who and how Buddha really was you can learn here